Disposal of waste vehicle tyres and tyre compositions present a significant challenge. Large numbers of waste tyres are simply sent to landfill sites. Alongside the environmental impact of discarding significant amounts of non-biodegradable material in landfill sites, this approach also presents a risk of accidental fires with the associated high pollutant emissions.
Traditional tyre compounds contain significant amounts of carbon black as a reinforcing agent. It is known to process these used tyres using a pyrolysis method, i.e. heating under an inert atmosphere. This pyrolysis releases gaseous products, that can be reclaimed and recycled in some cases, and also oils and liquid organic components that can also be recycled to some degree. The product is typically called “char” and is the remaining solid non-volatile components. This “char” contains a large proportion of carbon black, carbonaceous residues formed during polymer decomposition and an ash component comprising other non-volatile parts of the original tyre composition. The carbonaceous residues form on the surfaces of the originally compounded carbon black and inorganic particles, essentially fusing together large agglomerates. The carbonaceous residues reduce surface activity and dispersability of the pyrolysis carbon black, both of which have a negative impact on reinforcement. It is known in some cases to subject char to a further processing step to convert it into more valuable products such as activated carbon and porous carbon. For example Suuberg & Aarna (Carbon, 45 (2007) 1719-1726 describe a process for producing porous carbon materials from scrap automotive tyres using an oxidation process. Quek & Balasubramanian, Chemical Engineering Journal 170 (2011) 194-201 describe a post-pyrolysis oxidation process for enhancing absorption characteristics of pyrolytic char. U.S. Pat. No. 4,435,378 describes a process for removing extractable substances from carbon black using an oxidising gas treatment.
It is also known to treat waster tyres under partially oxidising pyrolysis conditions in a fluidised bed reactor to produce useful gaseous products (Lee et al. Energy, 20(10), 969-976 (1995).
However, it remains desirable to provide a method for recovering silica-containing material with a very low or zero carbon content from silica-containing rubber compositions.